California has become the headquarters of a new kind of stock exchange that trades personal data. The US firm BlueKai collects interests and preferences of over 200 million Internet users and sells them to the highest bidder.

Personal interests and preferences are divided into diverse categories that range from wedding dresses to mountain bike helmets. The information is very profitable for advertisers as it helps them to target potential customers showing more precisely.

For instance, an Internet user who makes an online search for a gadget will receive advertisements from a gadgets company within minutes of their first mouse click. The effectiveness of such advertising has transformed online data trading into a promising industry.

This multimillion-dollar industry is based on the clandestine business of spying on Internet users. However, a great number of Internet users are still unaware of its existence. Sophisticated tracking devices are deployed on numerous websites to sneakily gather personal interests and other information of the visitors.

Online tracking is performed almost completely via cookies and beacons that belong to companies other, and not to the original website.

Different studies revealed astonishing results. Australian-owned websites Ninemsn. com. au deployed 109 tracking devices, while Bigpond. com was found having deployed 93 tracking tools to gather visitors' interests and preferences.

Internet search giant Google reportedly tracks visitors on as many as 1 million websites where it displays its advertisements.